A vehicle may include a CVT to improve vehicle fuel economy and reduce vehicle weight. In some examples, the CVT may couple an engine to wheels via a belt or chain, and the belt or chain is positioned between a variable driving pulley and a variable driven pulley. A radius of the variable driving pulley (e.g., a pulley in the CVT closest to the engine along a torque path in the driveline arrangement) may be increased or decreased to change the CVT input to output ratio. Likewise, a radius of the variable driven pulley (e.g., a pulley in the CVT farthest from the engine along the torque path in the driveline arrangement) may be increased or decreased to change the CVT input shaft turns to output shaft turns ratio (e.g., the CVT ratio). The belt or chain may transfer engine torque from the engine to vehicle wheels while a driver of the vehicle is requesting positive torque. On the other hand, if the driver is not requesting torque or very little torque and the vehicle is traveling on a road having a negative grade, the belt or chain may transfer a portion of the vehicle's kinetic energy to the engine. Engine friction and pumping work (e.g., compression and expansion of gases within the engine) may oppose torque transferred from the vehicle's wheels to the engine so that vehicle acceleration may be reduced.
In other examples, the CVT may take form a planetary gear set and a generator. Torque of the generator may be adjusted so that engine speed may be controlled independently of wheel speed. Engine torque and torque of a motor positioned downstream of the generator in a vehicle driveline may be used to propel the vehicle when driver demand is high. Conversely, at least a portion of the vehicle's kinetic energy may be transferred through the planetary gear set and to the engine via adjusting a torque of the generator when driver demand is low and engine braking is requested.
However, when a CVT is transferring torque from vehicle wheels to an engine to utilize engine braking, driveline noise and vibration may increase to undesirable levels because the CVT may be controlled to limit engine speed to less than a sole threshold speed. For example, the CVT may control engine speed to remain less than a maximum engine speed, and the driveline may make a considerable amount of noise when the engine is rotating near maximum engine speed. Thus, it may be desirable to provide a way of utilizing engine braking while limiting exposure of vehicle occupants to driveline noise and vibrations.
The inventors herein have recognized the above-mentioned issues and have developed a vehicle operating method, comprising: operating a continuously variable transmission (CVT) via a controller to adjust engine speed according to a first engine speed to vehicle speed profile in response to a request for engine braking; and changing operation of the CVT via the controller to adjust engine speed according to a second engine speed to vehicle speed profile in response to a vehicle speed error and brake pedal position.
By adjusting engine speed via a CVT according to one of a plurality of engine speed to vehicle speed profiles, it may be possible to provide a desired level of engine braking without producing more driveline noise and vibration than is desired. One or more of the plurality of engine speed to vehicle speed profiles may hold engine speed constant while vehicle speed increases after the vehicle has reached a threshold speed. In addition, speed of an engine may be constrained to less than a threshold engine speed for each engine speed to vehicle speed profile, and each engine speed to vehicle speed profile may have a different upper threshold or maximum engine speed than other engine speed to vehicle speed profiles. The different engine speed to vehicle speed profiles may have different upper engine speed thresholds so that engine noise may be adjusted to the severity of road grade. In this way, driveline noise may scale with grade in a way that provides an improved vehicle driving experience. Further, the conditions for changing between the different engine speeds to vehicle speed profiles may provide for improved vehicle drivability by limiting switching between the profiles.
The present description may provide several advantages. In particular, the approach may provide desirable levels of driveline noise and vibration during engine braking in a vehicle that includes a CVT. Further, the approach may provide conditions that are useful to limit driveline noise according to severity of road grade so that a driver experiences a driveline noise level that may be more commensurate with driving conditions. In addition, the approach may provide additional engine braking after an engine reaches a threshold speed so that vehicle speed may be controlled without having to switch to a different engine speed to vehicle speed control profile.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.